Chicken Chow Mein with the Best Chow Mein Sauce

Ingredients

• 2 boneless, skinless chicken breasts
• 2 tablespoons vegetable oil
• 1 medium onion, thinly sliced
• 1 cup shredded carrots
• 1 cup sliced bell peppers
• 3 cups cooked chow mein noodles
• 2 cloves garlic, minced
• 2 tablespoons soy sauce
• 2 tablespoons oyster sauce
• 2 tablespoons hoisin sauce
• 1 tablespoon rice vinegar
• 1 teaspoon sesame oil
• 1 tablespoon cornstarch
• 1/4 cup water
• 1 teaspoon sugar
• 1/2 teaspoon ground white pepper
• 2 green onions, chopped
• 1/4 cup bean sprouts (optional)